1. Field of the Invention
The present invention relates to an MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spins of an object magnetically with an RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which acquire an MR (magnetic resonance) image of flowing matter by using SSFP (Steady State Free Precession).
2. Description of the Related Art
Magnetic Resonance Imaging is an imaging method which magnetically excites nuclear spins of an object set in a static magnetic field with a Larmor frequency RF signal and reconstructs an image based on NMR signals generated due to the excitation.
In the field of magnetic resonance imaging, the imaging method using SSFP (Steady State Free Precession) has been known. As a typical example of a high-speed imaging sequence using SSFP, there is a sequence referred to as TrueFISP (fast imaging with steady precession) (see, for example, U.S. Pat. No. 4,769,603).
FIG. 1 is a flowchart showing a conventional TrueFISP sequence.
As shown in FIG. 1, the conventional SSFP sequence such as the TrueFISP sequence applies an RF excitation pulse repeatedly at a constant and short TR (repetition time) with a same excitation angle (flip angle) α to lead magnetization in a steady state quickly. The gradient magnetic field is adjusted so that the zero-order moment (time integration) becomes zero. The gradient magnetic field in a read out axis direction is controlled so that the polarity inverts several times. As a result, an obtained echo signal has a high signal to noise ratio (SNR) and a signal intensity S depends on a relaxation time of a tissue as shown in the expression (1).S∝1/(1+T1/T2)  (1)
Note that, the expression (1) is a relational expression when an excitation angle α is 90 degrees. T1 and T2 are a longitudinal relaxation time of a tissue and a transverse relaxation time of a tissue respectively. As shown in the expression (1), the intensity S of signal obtained by the SSFP sequence depends on a relaxation time ratio T1/T2 of a tissue. Consequently, it is known that it is the most effective from the contrast viewpoint to regard a cine image of a heart as an applicable target of the SSFP sequence. In addition, the effectiveness of the SSFP sequence to imaging of the abdominal vasculature has been pointed out. Since blood vessels can be imaged without contrast medium by using the SSFP sequence, the SSFP sequence is receiving attention in the field of imaging of vessels.
In the meanwhile, the requirements needed for the SSFP sequence include requirements with regard to a phase of RF pulse in addition to the requirement that the zero-order moment of gradient magnetic field becomes zero as described above. The simplest control requirement with regard to a phase of RF pulse is that a phase of continuous RF pulse alternates between zero degree and 180 degrees (π radian).
FIG. 2 is a diagram showing a variation of magnetization intensity by a scan under the conventional SSFP sequence.
When the nutation angle is controlled so that each excitation angle of continuous RF pulses becomes α, and phase is controlled so that the phase of a continuous RF pulse alternates between zero and 180 degrees, the magnetization state alternates between the state (A) and the state (B) as shown in a vectorial representation in FIG. 2.
That is, phases of excitation pulses are controlled so that:
the excitation angle becomes α, α, α, . . . ,
the phase of excitation pulse becomes 0°, 180°, 0°, . . . , and
the state of magnetization becomes (A), (B), (A), . . . .
As shown in FIG. 2, a magnetization that reached a steady state becomes the state (A) that deviates from the static magnetic field direction by α/2. In this state (A) of magnetization, when an excitation pulse changing the phase by 180 degrees is applied, the magnetization state changes from the state (A) to the state (B). Moreover, in the magnetization state (B), when an excitation pulse changing the phase by 180 degrees is applied, the magnetization state returns from the state (B) to the state (A) again.
In this way, it turns out that a steady state is maintained effectively by changing the phase of continuous wave RF excitation pulses by 180 degrees. It is also known that the time required for transferring magnetization in thermal equilibrium to a steady state can be reduced by the foregoing phase control of an excitation pulse.
The SSFP sequence is also applied to imaging of a part where flowing matter such as blood flow exists but special consideration is made when flowing matter exists in an imaging area. That is, as shown in the expressions (2-1) and (2-2), a gradient magnetic field is controlled so that not only does a zero-order moment of the gradient magnetic field become zero, but a first-order moment of the gradient magnetic field becomes zero in order to prevent magnetization of matter flowing in the gradient magnetic field direction from a phase shift.∫Gdt=0  (2-1)∫Gtdt=0  (2-2)
wherein G denotes the intensity of the gradient magnetic field and t denotes time.
FIG. 3 is a diagram explaining the conventionally known phase shift.
FIG. 3(a) shows a gradient magnetic field to be applied. FIG. 3(b) shows a time variation of phase of magnetization in matter flowing in the direction in which the gradient magnetic field shown in FIG. 3(a) is applied.
The phase of magnetization of matter flowing in the gradient magnetic field direction changes due to the gradient magnetic field applied as shown in FIG. 3, and undergoes what is called a phase shift. For this reason, it is obvious that the steady state as shown in FIG. 2 is not maintained.
Consequently, in the conventional SSFP sequence, the gradient magnetic field is determined so as to avoid the foregoing phase shift and maintain a steady state.
FIG. 4 is a diagram showing a relationship between a gradient magnetic field applied by the conventional SSFP sequence and a phase of magnetization of matter flowing in the gradient magnetic field direction.
FIG. 4(a) shows a gradient magnetic field applied by a conventional SSFP sequence. FIG. 4(b) shows a time variation of phase of magnetization in matter flowing in the direction in which the gradient magnetic field shown in FIG. 4(a) is applied.
As shown in FIG. 4(a), when the gradient magnetic field is applied so that the zero-order and the first-order moments become zero, a magnetization of matter flowing in an application direction of the gradient magnetic field undergoes a phase shift as shown in FIG. 4(b). However, since the first-order moment of the applied gradient magnetic field is zero, the phase shift is offset as shown in FIG. 4(b) and the phase shift does not eventually occur in the magnetization of the flowing matter.
As described above, although both of the zero-order moments of the gradient magnetic field shown in FIG. 3 and FIG. 4 are zero, it turns out that a phase shift does not occur when the gradient magnetic field as shown in FIG. 4 is applied while a phase shift occurs when the gradient magnetic field as shown in FIG. 3 is applied. In other words, in the conventional SSFP sequence as described above, both the zero-order and the first-order moments of the gradient magnetic field need to be zero to avoid a phase shift in flowing matter.
For this reason, the conventional SSFP sequence is carefully assembled so as to satisfy the foregoing various conditions in order to avoid a phase shift in magnetization of flowing matter. As a result, magnetization of flowing matter such as blood flow can be imaged with a satisfactory SNR as well as magnetization of a static matter such as an organ.
However, an image obtained by imaging under the conventional SSFP sequence becomes an image where flowing matter such as blood flow overlaps with organs. For this reason, there is a problem that it might become difficult to distinguish blood flow and vessels from organs in case of focusing on only the blood flow and the vessels since the flowing matter and the organs coexist.
In addition, in the conventional SSFP sequence, there is a problem that a steady state of magnetization of flowing matter might not be maintained well when the control of the gradient magnetic field is inadequate although magnetization of a static matter such as an organ maintains a steady state effectively.
Therefore, a technique for imaging only a vessel and flowing matter such as blood flow by using the SSFP is required.